The invention relates to a Hall element and a method of manufacturing same which utilizes an evaporated semiconductor film.
Hall elements are widely used to detect or determine the magnitude of a magnetic field by utilizing the Hall effect. A conventional Hall element comprises a single crystal of a semiconductor material having a high mobility. The Hall effect can be expressed by the following formula: EQU E .varies. RBI/t
where E represents a Hall voltage developed, R the Hall coefficient of the material, B the applied flux density, I the current applied and t the thickness of the semiconductor employed. Heretofore, the trend has been to reduce t or the thickness of the single crystal of semiconductor used in order to maximize the output voltage from the Hall element. However, there is a limitation on the thinness which can be achieved, and the minimum thickness is 10 .mu.m under the present status of technology. Also difficulties are experienced in obtaining a single crystal of an increased area. When the material is cleaved to a reduced thickness, the cleaved crystal must be polished, resulting in a waste of the material. In view of these considerations, it has been proposed to form an evaporated thin film of a semiconductor having a high mobility. When a thin film is formed by evaporation, it is possible to achieve a thickness on the order of 1.5 .mu.m. However, to provide a satisfactory film with such a thickness, the unevenness of the surface of a substrate which is used for the evaporation process must be reduced to at least below 1.5 .mu.m. If the evaporated film is subjected to a photolithographic etching, the surface of the substrate on which the thin film is to be formed must be roughened to a suitable degree in order to avoid exfoliation of the evaporated film. Therefore, it will be appreciated that the requirement for a reduced thickness of the evaporated film and the demand on the evaporation substrate for the convenience of manufacturing and stability are conflicting. As will be understood from the above formula, the output can be increased by concentrating the magnetic flux to thereby increase the magnitude of B. This may be achieved by using a bulk element which is sandwiched between a pair of closely spaced magnetizable members. However, where the thin film is evaporated on a non-magnetic substrate, the magnetizable member can only be disposed on one surface of the film. Though it may appear that the semiconductor film can be directly evaporated on an electrically nonconductive, magnetizable member, the preparation of the magnetizable member to provide a sufficiently smooth evaporation surface is liable to cause exfoliation, resulting in difficulties during and after the manufacturing process. On the other hand, if a certain degree of unevenness of the evaporation surface is allowed, an evaporated film of sufficiently reduced thickness and having a satisfactory film quality cannot be obtained.
Considering some known Hall elements, British Pat. No. 1,015,469 describes the manufacturing of a Hall element which comprises preparing a single crystal of indium antimony to a thickness on the order of 0.01 inch (254 .mu.m), bonding it to a ferrite slab which is smoothly polished by means of epoxy resin, reducing the indium antimony to a thin film on the order of 0.0005 to 0.001 inch (12.7 to 25.4 micronmeter) by polishing with a parallel diamond lapper, and bonding another ferrite thereon. The process described in this patent employs the polishing of a semiconductor to a thin film, which cannot be reduced to a thickness less than 10 to 30 microns which represent the limit achievable with the polishing technique. As a consequence, a Hall element having a high output cannot be manufactured. Additionally, there is a waste of material because of the use of the bulk element.
British Pat. No. 926,250 describes slicing a single crystal of indium antimony semiconductor to a thickness less than 250 micrometer or even below about 60 micronmeter, and holding it between a pair of ferrite members with epoxy resin to form a Hall element. However, a high output element cannot be obtained with a thin film of such a thickness.
British Pat. No. 1,017,033 describes alternating evaporated layers of Hall effect material such as indium antimony or the like with layers of ferromagnetic material, which is sprayed, coated or applied on a plastic film for the purpose of information recording and retrival. This patent discloses that the thickness of the ferromagnetic layer is from 0.02 to 1 microninches and that the Hall effect layers comprising indium antimony having the same thickness. Such thickness is from 5.1 Angstrom (0.00051 micronmeter) to 255 Angstrom (0.0255 micronmeter). When the thickness is extremely reduced below 0.5 millimicron, the surface effect degrades the characteristic, preventing a high output from being produced. In an experiment conducted by the inventors, it is found that the evaporation of a thin semiconductor film directly on a ferromagnetic material cannot achieve a satisfactory output. This is considered to be due to the failure of the ferromagnetic material to provide sufficiently smooth surface. U.S. Pat. No. 3,082,124 discloses operating parameters such as temperature, pressure or the like which are employed during an evaporation process of a thin film of indium antimony semiconductor onto a glass plate or ceramic plate. However, this patent does not teach useful configuration of holding it between ferrite members. A satisfactory flux concentration cannot occur when the thin film is held between magnetizable members with an evaporating substrate interposed therebetween.
German Pat. No. 1,098,581 describes a semiconductor element having reluctance effect, which is encapsulated in an insulating material together with leads attached thereto, which is in turn secured between a pair of ferrite members. While not specifically described, it is undoubtedly clear from the illustration and the technical level in 1955 when that invention was made that the element comprises a single crystal which cannot have a thickness less than 10 micronmeter.
From the foregoing discussions, it will be appreciated that a considerable amount of effort has been directed to obtaining a thin film of semiconductor having a high mobility, but failed to produce a thin film of satisfactory quality and having a thickness on the order of 0.5 .mu.m to 1.5 .mu.m. As a result, with the prior art Hall elements, the activating current must be increased to result in an increased current dissipation. In addition, the signal-to-noise ratio is low, preventing their use to the detection of a very weak magnetic field.
It is an object of the invention to provide a Hall element using an evaporated film of a semiconductor having a high mobility and which is of a sufficiently reduced thickness and has a sufficiently large flux concentration effect to produce a high output.
It is another object of the invention to provide a Hall element which produces a high output and provides a good stability and a long useful life.
It is a further object of the invention to provide a Hall element capable of producing a high output and which exhibits a good resistance to even a high humidity environment.
It is an additional object of the invention to provide a method of manufacturing a Hall element which minimizes the quantity of the material used and which permits a high output Hall element to be produced inexpensively.
It is still another object of the invention to provide a practicable method of manufacturing a Hall element which produces a thin film of a high mobility semiconductor of a reduced thickness and having an improved crystal structure.